Update project to use Node.js 18 or 20 LTS (16 was end of life September 2023)

[jeromecovington]

Apologies if this has already been discussed, but I noticed that the .nvmrc and GitHub workflows are using Node.js 16 which was end of life September 2023. I would recommend increasing the version to 18 which is the newest version in maintenance mode and will reach end of life in April 2025. As another alternative, the project could be upgraded to use version 20 LTS which will transition to maintenance mode and reach end of life in April 2026. I am happy to put the PR together, but not being intimately familiar with this project I wasn't sure if it was pinned to Node.js 16 for some already discussed reason.

For more information on the Node.js version matrix, please see: https://github.com/nodejs/Release